The present invention relates generally to ion implantation systems and more particularly to a mechanism for preventing the generation of neutron radiation within the beamline of such systems, especially high-energy ion implantation systems.
Ion implantation has become the technology preferred by industry to dope semiconductors with impurities in the large-scale manufacture of integrated circuits. High-energy ion implanters are used for deep implants into a substrate. Such deep implants are required to create, for example, retrograde wells. Eaton GSD/HE and GSD/VHE ion implanters are examples of such high-energy implanters. These implanters can provide ion beams at energy levels up to 5 MeV (million electron volts). U.S. Pat. No. 4,667,111, assigned to the assignee of the present invention, Eaton Corporation, describes such a high-energy ion implanter.
Ion implanters operate at very high voltage levels. Typically, ions in the beam are accelerated and decelerated by electrodes and other components in the implanter that reside at differing voltage levels. For example, positive ions are extracted from an ion source, passed through a mass analysis magnet, and accelerated by electrodes having increasingly negative potentials. In a high-energy ion implanter, the ion beam accelerates as it passes through a radio frequency (RF) linear accelerator (linac). The ion beam progresses through the RF linac by passing through a series of acceleration stages (resonator modules) in which accelerating fields are produced by synchronizing the frequency of the RF voltage to the ion beam velocity.
Arsenic (As) and phosphorous (P) are two species that are often implanted in semiconductors as doping agents. Arsenic and phosphorous are typically injected into the ion source ionization as arsine (AsH3) and phosphine (PH3) gas, respectively, each of which includes hydrogen (H) as a carrier gas. Ionization of these gases within the ionization chamber often produces small amounts of deuterium, an isotope of hydrogen. The ion beam that is extracted from the ion source often includes deuterons, which are nuclei of deuterium atoms, each comprising a proton and a neutron. The deuterons are thus subatomic particles having a unit positive charge.
The deuterons extracted from the ion source are transported along with the ion beam to the mass analysis magnet. The implantable ions having the correct charge-to-mass ratio (e.g., As+, As++, P+ and P++) pass through the mass analysis magnet and the particles having incorrect charge-to-mass ratios (e.g., deuterons) impact the interior sidewalls of the mass analysis magnet. These sidewalls are often lined with strike plates, typically constructed using graphite, which is a hexagonally crystallized allotrope of carbon (C).
Of the known isotopes of carbon, C-11 through C-15, C-12 and C-13, both of which are stable, are the most abundant. The number 12 in the C-12 (or 12C) designation represents the sum of the protons (6) and neutrons (6). Slightly less than 99% of the carbon isotopes found on earth are carbon C-12. Carbon C-13 (or 13C) has 7 neutrons and 6 protons in its nucleus. Approximately 1.1% of all carbon atoms are made of this isotope.
The graphite used to create mass analysis magnet strike plates generally comprises both carbon C-12 and smaller amounts of carbon C-13 isotopes. However, if deuterons impact the strike plate, a nuclear reaction may occur involving the incident deuteron and the nucleus of the C-13 atom within the composite C-12 and C-13 graphite structure. Such nuclear reaction may release a neutron with significant energy (up to approximately 5 MeV). Radiation of these neutrons from the surface of the strike plate is undesirable. Neutron radiation caused by deuteron collisions with carbon C-13 isotopes in strike plates is especially problematic in high-energy ion implanters, where high beam energies can increase the generation of neutron radiation.
It is an object of the present invention, then, to provide an ion implanter beamline that prevents or minimizes the generation of neutron radiation during operation of the implanter. It is a further object to provide such a beamline in the form of an improved mass analysis magnet strike plate.
A mass analysis magnet assembly is provided for use in an ion implanter, comprising: (i) a magnet for mass analyzing an ion beam output by an ion source, the magnet providing an interior region through which the ion beam passes; and (ii) at least one strike plate in part forming an outer boundary of the interior region. The strike plate is comprised of an isotopically pure carbon-based material. The isotopically pure carbon-based material, preferably by mass greater than 99% carbon C-12, prevents neutron radiation when impacted by deuterons extracted from the ion source. The strike plate may comprise an upper layer of isotopically pure carbon C-12 isotope positioned atop a lower substrate.